Processes such as a skew correction process, a noise removal process, a distortion correction process, and the like are carried out in an image processing apparatus for processing a document image read out by an image reading apparatus such as a scanner or the like. The skew correction process is a process for detecting whether the document is skewed or not based on the read image, and correcting document skew in the read image. The noise removal process is a process for removing isolated point noises present in the image. The distortion correction process is a process for correcting, in a case where the document is of a bound book, distortion in an image of a binding section.
For the skew correction process, a Patent Literature 1 below discloses a technique in which a contour of a document is extracted, and document skew (correction of the document skew by an image process) is corrected based on the extracted contour.